The present invention relates to a method for producing an optical fiber cable in which optical fibers are respectively disposed in a plurality of grooves formed in the outer surface of a spacer.
A conventional method as disclosed in Japanese Patent Unexamined Publication No. 55-12992 is depicted in FIGS. 4 through 6. Referring to FIG. 4, there is shown a spacer 1 for holding a plurality of optical fibers separately from each other. A plurality of grooves 2 are formed in the outer surface of the spacer 1 circumferentially separately from each other. There are four such grooves depicted in FIG. 4, although the spacer may have more or less. The grooves 2 extend over the whole length of the spacer. Each optical fiber is fit into a respective groove. Each of the grooves extends in the shape of SZ twist, that is, changing direction. The respective changes of direction of the four grooves 2 are made at positions longitudinally in the same cross-section of the spacer 1. Further, the four grooves extend substantially parallel to each other.
FIG. 5 is a diagram of a typical apparatus used for producing an optical fiber cable. The apparatus is provided with an extruding unit 3, a twisting unit 4, and a fitting unit 5. In producing an optical fiber cable, first, a spacer 1 including a tension member 26 therein is supplied into the extruding unit 3 which is provided with a die (not shown). The die has a shape capable of forming grooves 2 in the outer surface of the spacer 1 when the spacer 1 comes out of the extruding unit 3. The twisting unit 4 twists the spacer 1 so that the directions of the grooves 2 are changed when the spacer 1 comes out of the twisting unit 4.
The fitting unit 5 fits optical fibers 7 pulled out from respective reels 6 into the respective grooves 2 of the spacer 1. FIG. 6 shows the fitting unit 5 in detail.
As shown in FIG. 6, two guide plates 8 and 9 are arranged so as to surround the spacer 1. The guide plate 8 has four fixed tubes 10. The forward ends of the respective tubes 10 are arranged so that they may be fitted in the respective grooves 2. The optical fibers 7 pulled out from the respective reels 6 are fit into the respective grooves 2 of the spacer 1 through the two guide plates 9 and 8 and further through the respective tubes 10. When the spacer 1 is continuously longitudinally moved, the tubes 10 move along the paths of the respective grooves 2. Since the tubes 10 are fixedly attached to the guide plates 8, the guide plate 8 performs reciprocating rotation in synchronism with the displacement of the grooves 2 of the spacer 1.
The above apparatus and method, however, has the following problem. Because the guide plate 8 for guiding the optical fibers 7 is connected to the grooves 2 of the spacer 1 through the tubes 10 so that the guide plate 8 rotates in synchronism mechanically with the displacement of the grooves 2 of the spacer 5 where the period of change of directions of the grooves 2 of the spacer 1 is short, the influence of inertia force of the guide plate 8 is large. This inertia causes large side pressure which acts on the respective side surfaces of the grooves 2 of the spacer 1. Therefore, the respective side surfaces of the grooves 2 of the spacer 1 are sometimes damaged.